A variety of techniques are established in the art for analysing both the elemental and structural characteristics of a material having a crystalline structure. For example, x-ray diffraction (XRD) relies upon analysis of the pattern produced by diffraction of x-rays through the closely spaced lattice of atoms in a crystal to reveal the structural constituency of the analysed material. Bragg's law allows the spacing in the crystal lattice to be inferred from the measured path difference for diffracted x-rays.
X-Ray fluorescence (XRF), by contrast, is a spectroscopic technique to allow elemental investigation of a sample without the need for chemical analysis. In XRF, illumination of a sample with an x-ray beam results in emission of secondary x-rays having characteristic wavelengths which are indicative of the elemental constituency of the sample. In order to permit multi elemental analysis, the x-ray source for XRF is typically polychromatic.
Combined XRD/XRF instruments have existed for many years. A first type of combined XRD/XRF instrument operates with the sample at atmospheric pressure. A second type of combined instrument operates in vacuo. Each type has advantages and disadvantages: instruments wherein the sample is analysed in a vacuum tend to provide an enhanced x-ray analysis particularly though not exclusively for XRF where the sensitivity to elements having a low atomic number is increased. On the other hand, the size and physical arrangement of a non-vacuum instrument is less constrained, and moreover changing of samples can be carried out more promptly.
For high quality XRD and a more complete structural characterisation for mineralogy and phase analysis, it is desirable to be able to change the measurable angle of diffraction through a wide range. In a non-vacuum system this does not present too much difficulty. However in a vacuum chamber the restricted space limits the opportunity to improve performance.
Several solutions to the problem of limited space when analysing a sample in a vacuum chamber using XRD techniques are proposed in the art.
In XRD-only devices, the x-ray tube and detector may be rotated with the sample fixed. For combined XRD/XRF devices however, a single x-ray tube is held in a fixed location relative to the vacuum chamber and the sample is rotated whilst the detector is held fixed, the sample is held fixed whilst the detector is rotated, or, as in U.S. Pat. No. 4,263,510, U.S. Pat. No. 5,369,275 and U.S. Pat. No. 4,916,720, the sample and detector are both rotated. The latter arrangement appears to provide the highest performance in vacuo.
For high quality XRF, however, the distance between the sample and the tube needs to be small. Unfortunately this requirement forces a compromise in a combined XRD/XRF instrument since, as noted, the highest quality XRD measurements require the sample to be rotatable. This in turn puts a minimum distance requirement on the location of the x-ray tube relative to the sample (to avoid collisions between the two during XRD measurements), so reducing the maximum performance during XRF measurements.
Commonly assigned U.S. Pat. No. 5,406,608 describes a combined XRD/XRF analyser for analysing samples in vacuo. An x-ray source is mounted in fixed relation to a vacuum chamber of the instrument and provides a polychromatic divergent x-ray beam which illuminates a sample to permit both XRD and XRF measurements. One or more fixed and/or moveable fluorescence channels are provided so as to allow selection of x-rays of a particular wavelength and energy, and to detect the selected x-ray. A diffraction channel is also provided which allows selection of a characteristic x-ray wavelength at the source following diffraction by the sample. The diffraction channel also has a detection arrangement. The x-ray diffraction detector is rotatable to improve XRD measurements. XRF performance is however optimised by providing multiple fluorescence channels or by mounting a fluorescence channel (incorporating a detection arrangement) on a goniometer rotatable about the sample.
Whilst the foregoing arrangement provides a fair compromise between XRD and XRF performance, it does however suffer from a number of drawbacks. Firstly, the sample is fixed relative to the x-ray tube (in other words only the XRD detector arrangement is rotatable, not the sample) which limits XRD performance. Secondly, in seeking to avoid compromising the XRF performance, the arrangement of tube, sample detectors and vacuum chamber in U.S. Pat. No. 5,406,608 restricts the angular range of the XRD detector which in turn limits the ability to perform more extensive XRD measurements.